The long-term objective of this research is to advance understanding of how positive emotions enhance health, mental health, and social relations. Fredrickson's broaden-and-build theory of positive emotions provides the foundation. The theory states that, unlike negative emotions, which narrow people's ideas about possible actions (e.g., fight, flee), positive emotions broaden people's thought-action repertoires (e.g., play, explore), and build their enduring personal resources (e.g., health, resilience, social support, intellectual complexity). Nine studies target four Specific Aims. Studies 1-2 target Specific Aim I: To investigate and test the role of positive emotions in flourishing mental health. Study 1 uses a new survey method to assess whether flourishing mental health is associated with high ratios of positive to negative emotions. Study 2 uses a between-subjects experimental design and a new self-regulation intervention strategy to test whether increasing daily experiences of positive emotions increases symptoms of flourishing and decreases symptoms of depression. Studies 3-4 target Specific Aim I1: To test whether and how positive emotions build consequential and lasting personal resources. Study 3 uses a between-subjects experimental design and the same self-regulation intervention strategy to test whether increasing daily experiences of positive emotions builds resilience, which in turn alters people's emotional, cognitive, and physiological functioning. Study 4 tests whether cognitive broadening, assessed through linguistic markers, accounts for the building effects of positive emotions. Studies 5-6 target Specific Aim II1: To test whether and how positive emotions broaden interpersonal curiosity and build high quality connections between people. Each uses a between-subjects experimental design to induce emotions (joy, anger, or a neutral state) and test whether positive emotion increases people's curiosity about a new acquaintance, and thereby expands interpersonal understanding and empathy, and improves the ways individuals relate to one another. Studies 7-9 target Specific Aim IV: To test whether and how positive emotions build efficient social coordination and effective teamwork. Each uses a between-subjects experimental design to induce emotions (joy, anger, or a neutral state) and test whether positive emotion increases perceptions of group identity and thereby helps teams coordinate and economize their use of resources.